In a general aspect thereof, the present invention relates to a cable to be used to transmit electric current in conditions of so-called superconductivity, i.e. in conditions of almost null electric resistance.
More particularly, the invention relates to a superconducting cable comprising:
a) a layer of tapes comprising superconducting material,
b) a tubular element for supporting said layer of tapes comprising superconducting material
c) a cooling circuit adapted to cool the superconducting material to a working temperature non higher than its critical temperature.
In the following description and the subsequent claims, the term: superconducting material, indicates a material, such as for instance special niobium-titanium alloys or ceramics based on mixed oxides of copper, barium and yttrium, or of bismuth, lead, strontium, calcium, copper, thallium and mercury, comprising a superconducting phase having a substantially null resistivity under a given temperature, defined as critical temperature (in the following also shortly referred to as Tc).
The term: cable for high power, indicates a cable to be used for transmitting current quantities generally exceeding 3,000 A, such that the induced magnetic field starts to reduce the value of the maximum current density achievable in superconductivity conditions.
The term: superconductor cable indicates in the following any element capable of transmitting electric current in superconductivity conditions, such as for example tapes of superconducting material wound onto a supporting core.
The superconducting cables comprise a structural element consisting of the supporting tubular element of the superconducting material.
Patent application EP 97202433.5 in the name of the Applicant discloses a supporting tubular element entirely consisting of a tube made of polymeric material, typically polytetrafluoroethylene or polyamide.
The same patent application EP 97202433.5 also discloses a supporting tubular element made of metallic material such as steel, copper or aluminum.
The superconducting cable is installed at room temperature, as well as the electrical (to the terminals) and hydraulic connections (attached to the cooling circuits of the cable).
After the installation the cable is brought to its working temperature by means of the cooling liquid. During such cooling each component of the cable is submitted to mechanical stresses of thermal nature, according to the thermal coefficients of the constituting materials.
In particular mechanical stresses are generated in the layers of superconducting materials and at the terminals connected to the ends of the cable.
The Applicant has noticed that the supporting element must not only offer a satisfactory mechanical support to the layer or layers of superconducting material, but also at the same time perform a number of additional functions not less important for the good operation of the cable.
More particularly, the supporting element should:
i) ensure that during cooling of the cable no internal stresses are generated within the superconducting material nor at the ends of the cable;
ii) ensure the mechanical stability of the cable, that is to say, the cable can be bent according to bending radiuses compatible with the diameters of the reels onto which the cable is wound for its transport;
iii) contribute to the mechanical resistance of the cable during the installation; and
iv) substantially contribute to the cryostabiliity of the cable in case of short circuit, this term indicating both keeping the superconducting material below its critical temperature and keeping the cooling fluid in liquid state.
The Applicant has found that the use of a substantially composite supporting tubular element allows to reduce the stresses imparted to the superconducting material both in radial direction and along a longitudinal direction, while ensuring at the same time a sufficient amount of metallic material for ensuring the cryostability of the cable.
According to a first aspect the invention relates to a superconducting cable of the above indicated type, which is characterized in that said tubular element is composite and comprises a predetermined amount of a first material having a first thermal expansion coefficient and a second material having a thermal expansion coefficient higher than that of said first material, said thermal expansion coefficients and said amounts of said first and second material being predetermined in such a way that said tubular element has an overall thermal shrinkage between the room temperature and said working temperature of the cable such as to cause a deformation of said tapes comprising superconducting material lower than the critical deformation of the same tapes.
In a second aspect thereof, the invention relates to a superconducting element characterized in that said tubular element is substantially composite and comprises a electrical contact with the layer of superconducting material and at least one second polymeric material associated to said first material.
According to a third aspect of the invention, a method for limiting the tensile stresses along a longitudinal direction imparted to opposite fixing terminals of a superconducting cable of the type with clamped heads as a consequence of cooling is provided, the cable comprising at least one layer of superconducting material, which is characterized by providing in the cable a composite tubular element for supporting the layer of superconducting material.
In the following description and in the subsequent claims, the term: superconducting cable of the type with clamped heads, indicates a cable whose opposite ends are mechanically constrained to respective fixing terminals in such a way that no substantial relative sliding in axial direction between tapes and supports and with respect to the terminal themselves takes place.
Advantageously, the aforesaid composite supporting tubular element is able not only to adequately support the superconducting material, but also to limit the stresses induced along a longitudinal direction in the layer of superconducting material and in the terminals connected to the ends of the cable and to provide at the same time an amount of metal in electrical connection with the superconducting material, capable to substantially contribute to the cryostability of the cable during the short circuit transient.
In particular, it has been found that such composite supporting tubular element, thanks to the presence of the above indicated second material having a higher thermal expansion coefficient, has an overall thermal expansion coefficient equal to or higher than that of the superconducting material, and therefore during the cooling step of the cable is able to shrink in radial direction to a greater extent with respect to entirely metallic supports, or, anyway, to an extent such as not to cause unacceptable deformations in the tapes.
In this way, the composite support according to the invention allows a greater shrinkage thereof along a longitudinal direction and, hence, allows to reduce the stresses along a longitudinal direction within the superconducting material due to the so-called constrained shrinking by the clamped heads.
Additionally, the use of a composite supporting tubular element advantageously allows to reduce in a substantial way also the stresses exerted along a longitudinal direction by the ends of the superconducting cable on the terminals with respect to the tubular elements entirely made of metal whenever the second material of the composite supporting tubular element also has a Young""s modulus (E) lower than that of the first metallic material.
The longitudinal stresses to which the supporting element of the cable is submitted in operation, in fact, are proportional to the product of the thermal expansion coefficient and the respective Young""s modulus (E) of the material which constitutes the supporting tubular element.
In contrast to the tubular element entirely made of polymeric material, furthermore, the composite tubular element of the invention allows to have in any case an amount of normal conductor in electrical connection with the superconducting material, which is sufficient for ensuring the cryostability of the cable during the short circuit transient.
For the purposes of the invention, the first metallic material of the composite supporting element is a metal preferably having a resistivity at 77 K less than 5*10xe2x88x929 xcexa9m, a specific heat at 77 K greater than 106 J/m3K and a heat conductivity at 77 K greater than 5 W/mK.
In particular, the first metallic material of the composite supporting element is selected from the group comprising: copper, aluminum and alloys thereof.
Preferably, the aforesaid second material is a non metallic material and has a thermal expansion coefficient higher than 17*10xe2x88x926xc2x0 C.xe2x88x921, preferably higher than 20*10xe2x88x926xc2x0 C.xe2x88x921, and still more preferably comprised between 40 and 60*10xe2x88x926xc2x0 C.xe2x88x921 .
In a preferred embodiment, the aforesaid second non metallic material is a plastics material.
For the purposes of the invention, the plastics material is preferably selected from the group comprising: polyamide, such as for example nylon, polytetrafluoroethylene (PTFE), polyethylene.
The values of the percent thermal shrinkage (F) between the room temperature and 77K and of the Young""s modulus (E) at 77K of some materials provided for use when manufacturing the composite supporting element according to the invention, are indicated in the following table.
In an advantageous embodiment, the aforesaid first and second materials are formed as adjacent annular sectors. Such design allows, in particular, to facilitate the step or manufacturing the composite tubular element.
For the purposes of the invention, the number of sectors of said first and second material and the arrangement of such sectors may be easily determined by a man skilled in the art on the basis of the construction requirements of the cable. For example, if a particularly high thermal shrinkage is required, the metallic portion may be reduced, for example down to 10% or less, while if greater stiffness or specific electrical characteristics are required, the polymeric portion may be consequently reduced.
Preferably, the number of sectors for manufacturing a composite supporting tubular element is comprised between 3 and 50. In a preferred embodiment, such number is chosen as a function of the outer diameter of the composite supporting tubular element and of the thickness of the sectors in such a way that the ratio xe2x80x9cKxe2x80x9d between the thickness xe2x80x9csxe2x80x9d of the sector and its width xe2x80x9clxe2x80x9d is comprised between 0,4 and 0,7.
Preferably, the sectors of said first and second material are alternately arranged one after the other. Such arrangement allows in fact to make a supporting tubular element having mechanical characteristics as homogeneous as possible which allow to ensure both a satisfactory dynamic stability of the stranding machine used for manufacturing the supporting tubular element, and the mechanical congruence of the composite supporting tubular element as a whole during the cooling of the cable.
Preferably, the annular sectors of said first and second material are spirally wound with a winding-angle comprised between 5xc2x0 and 50xc2x0. In such a way, it is possible to ensure a satisfactory and lasting clamping between adjacent sectors.
According to a further embodiment, the composite supporting tubular element of the superconducting material may comprise an inner tubular element essentially consisting of said second material onto which thin foils or wires essentially consisting of said first metallic material are wound.
Preferably the phase conductor comprises at least one superconducting tape wherein said layer or superconducting material is incorporated within a metallic coating.
Advantageously, the cable of the invention comprises a plurality of superconducting tapes spirally wound on the surface of the supporting tubular element according to a winding angle comprised between 5xc2x0 and 60xc2x0, and preferably between 10xc2x0 and 40xc2x0. In such a way, it is advantageously possible to further reduce possible mechanical stresses generated inside each of the aforesaid tapes.
According to an alternative embodiment, the phase conductor comprises at least one reinforcing foil of metallic material coupled, preferably in a substantially irreversible way, to the metallic coating of the superconducting tape and in electrical connection with the superconducting material.
In this way, during the short circuit transient, the overcurrent is split up between the metallic material of the tape, the metallic material of the supporting tubular element and the reinforcing foil, electrically connected in parallel to the superconducting material and constituting a resistive type conductor, by-passing the latter. At the end of the short circuit transient, the current may be transported again by the superconducting material in superconductivity conditions.
In particular, in the conductive element the electrical connection of the metallic material of the tape with the one hand, and with the reinforcing foil on the other hand, is made either placing the aforesaid materials in direct contact with one another or interposing between them conductive elements known per se.
Preferably, the reinforcing foil has a thickness not higher than half of the thickness of the metallic coating and advantageously contributes to increase the resistance of the conductive element of the cable at the various mechanical or thermal stresses, imparted thereto during installation or use.
Still more preferably, such thickness is comprised between 0.03 and 0.08 mm.
In a preferred embodiment of the invention, the resistance of the conductive element of the cable to the various stresses imparted thereto may be advantageously further increased submitting the superconducting material to a predetermined prestress degree along a longitudinal direction.
Such a prestress is preferably obtained by coupling the reinforcing foil to the coating material of the tape of superconducting material, while simultaneously applying to the foil a tensile stress substantially oriented along a longitudinal direction.
Advantageously, it has been found that such a prestress of the superconducting material is able to partially compensate the tensile effect applied on the superconducting material in the clamped heads arrangement of the cable when the latter is cooled from room temperature to the temperature of the cooling fluid.
Preferably a conductive element provided with reinforced tapes of the above mentioned type is obtained by applying a tensile stress comprised between 3.4*107 Pa (3.5 kg/mm2) and 34.3*107 Pa (35 kg/mm2) to the reinforcing foil by means of apparatuses known per se, such as for example by means of two coils, one for winding and the other for unwinding, of which one is suitably braked.
Due to such tensile stress, the superconducting material of the reinforced tapes so obtained has a % prestress degree along a longitudinal direction or xe2x80x9cxcex3xe2x80x9d, defined as follows:
xcex3=[(Lixe2x88x92Lf)/Li]* 100 
wherein:
Li=initial length of the tape;
Lf=final length of the tape after prestress;
comprised between 0.05 and 0.2%.
Preferably, the phase conductor comprises two reinforcing foils made of metallic material coupled to opposite faces of the metallic coating.
Preferably, the reinforcing foil and the metallic coating are reciprocally coupled in a substantially irreversible way by means of welding or brazing and in such a way as to ensure that the desired prestress of the superconducting material be maintained once the coupling is made. Advantageously, the desired electrical contact between the reinforcing foil and the metallic coating of the superconducting material is automatically ensured in case of coupling by means of welding or brazing.
Preferably, the reinforcing foil or foils and the metallic coating of said at least one superconducting tape consist of a metal selected from the group comprising: copper, aluminum, silver, magnesium, nickel, bronze, stainless steel, beryllium and alloys thereof.
Still more preferably, the reinforcing foil or foils coupled to the metallic coating of the superconducting tape or tapes consist of a metal selected from the group comprising: stainless steel, preferably magnetic, bronze, beryllium, aluminum, and alloys thereof, whereas one metallic coating consists of a metal selected from the group comprising: silver, magnesium, aluminum, nickel, and alloys thereof.
The superconducting cable of the invention may be both a coaxial and a non-coaxial cable.
In the following description and in the subsequent claims, the term: coaxial cable, indicates a cable comprising a supporting tubular element, a phase conductor coaxially surrounding the supporting tubular element, a layer of dielectric material external to the phase conductor and a return conductor supported by the layer of dielectric material and coaxial to the phase conductor.
For the purposes of the invention, inside the return conductor a current flows which is equal and opposite to that circulating inside the phase conductor, so as to generate a magnetic field equal and opposite to that generated by the current circulating in the phase conductor, so as to confine the magnetic field in the portion of the cable comprised between the two conductors and reduce the presence of dissipative currents in the cable portions externally supported with respect to the return conductor.
Preferably, the return conductor comprises at least one superconducting tape including a layer of superconducting material incorporated within a metallic coating and a predetermined amount of metallic material (stabilizing metal) in electrical contact with the metallic coating and having the function of allowing the stabilization of the superconducting material in short circuit conditions.
Preferably, besides, the overall amount of the stabilizing metal is determined by applying the same criterion of foil and adiabatic stability which is applied for the phase conductor and which will be reported in the following description.
Preferably, the stabilizing metal is split up in a plurality of straps or tapes, having a thickness comprised between 0.1 and 5 mm, in direct contact with the metallic coating of the superconducting tape, for example wound thereon.
In an alternative embodiment, the return conductor may comprise at least one metallic reinforcing foil coupled, preferably in a substantially irreversible way, to the metallic coating of the superconducting material and interposed between the latter and the stabilizing metal.
Similarly to what happens to the phase conductor, if the return conductor looses its superconducting capacities during the short circuit transient and the current passes through the stabilizing metallic material, the reinforcing foil (if present) and the metallic coating of the tapes (if present), to flow back in the superconducting material at the end of the short circuit.
Conveniently, the stabilizing metal of the return conductor, externally placed with respect to the superconducting tapes, may be split up in straps or wires, for example of copper or other suitable metal, associated to the superconducting tapes and, as such, also being spirally wound as the same tapes.
Preferably, the superconducting cable of the invention is cooled by means of a suitable pressurized and undercooled cooling fluid, in such a way as to ensure the heat exchange necessary for the operation of the cable and ensure that a temperature suitably lower than the critical temperature of the superconducting material is maintained, also for high lengths of the cable.
During its flowpath, in fact the cooling fluid is simultaneously submitted both to an increasing heating, as a result of the heat absorbed by the elements which constitute the cable, and to an increasing loss of pressure, due to the hydraulic losses while passing through the cable and to the more or less turbulent flow of the cooling fluid itself.
The choice or the working conditions of the cable is therefore made taking such phenomena into account. In particular, working conditions are preferred which maintain the cooling fluid far away from the temperature and pressure values of its own curve of saturation. Such working conditions are comprised inside the so called xe2x80x9cworking windowxe2x80x9d which delimits a portion in the state diagram of the cooling fluid inside which safety conditions exist with respect to the need of cooling the superconducting material below its critical temperature while maintaining the cooling fluid in liquid state.
Advantageously, the use of pressurized and undercooled cooling fluid allows, furthermore, to reduce the amount of metallic material employed as stabilizing metal.
Preferably, the superconducting material is of the so called xe2x80x9chigh temperaturexe2x80x9d type (Tc of about 110K) and is cooled to a temperature comprised between about 63K and 90K.
Such cooling is preferably achieved using liquid nitrogen as cooling fluid at a working pressure comprised between 10 and 20 bar.
According to the invention, the embodiments of the previously described superconducting cable may be various. In particular and as illustrated above, the cable of the invention may be coaxial or non-coaxial, the phase or the three existing phases may be monoelement or multielement, the electrical insulation may be both in cryogenic environment (cold dielectric) or at room temperature (warm dielectric), the thermal insulation may be made on each singles phase or on three joined phases.